In OFDM systems, the transmitter typically sends pre-determined values on specific time-frequency resource elements (tones) to enable channel estimation at the receiver. These pre-determined sequences, known as reference signals or pilot sequences, may be used by the receiver for chancel estimation, synchronization, noise covariance estimation and other purposes.
Assuming that tone k during OFDM symbol l is set aside for reference signaling, the transmit vector xkl is fixed beforehand and is therefore known to the receiver. Consequently the receiver uses the received signal on the tone, given byykl=Hklxkl+nkl  (1)to estimate the channel Hkl, the variance of the noise nkl and other related quantities. The exact sequences {xkl} to be transmitted on these reference signal tones is important for two reasons. First, they need to have sufficient randomness to ensure desirable properties like low peak-to-average-power ratio at an IFFT output. Further, they need to also be unique to each transmitter, to enable the receiver to differentiate one transmitter from another. Improvements in this area would prove beneficial in the art.